Impaired activation of glycogen synthase in insulin-resistant subjects is associated with both chronically lower activity of protein phosphatase-1 (PP1) and a smaller magnitude of activation of PP1 in response to insulin compared to that seen in insulin-sensitive subjects. Although PP1 activity is low in muscle of resistant subjects, the concentration of catalytic subunit of PP1 (PP1-C) is higher than in sensitive subjects (B. Nyomba and D. Mott, CDNS, PECRB). The predicted amino acid sequence of the catalytic domain of PP1 was examined in insulin sensitive and resistant subjects, using polymerase chain reaction (PCR), and found to be identical. PP1-C RNA concentration and size in skeletal muscle (1.6 kb) were also the same in fasting subjects of different insulin sensitivities. Therefore, it is unlikely that a major mutation in PP1 accounts for abnormal properties of PP1 activity in insulin resistance. The concentration of RNA corresponding to PP1-C in human skeletal muscle decreased by 50% after 30 min of a high dose infusion of insulin in vivo. The concentration returned to basal levels by 120 min. In contrast, PP1-C RNA increases slightly in response to insulin in insulin-resistant subjects.